To meet commercial, scientific, and military requirements in space, improved earth-to-orbit transportation must be developed. Any new vehicle should have low development costs, low operational costs, and increased performance over existing earth-to-orbit vehicles.
Of critical importance in the design of earth-to-orbit vehicles is the choice of fuel. Two parameters are useful in deciding which fuel to use. One parameter is the fuel specific-impulse, which is defined as the pounds of thrust that can be obtained per pound per second of propellant flow. The other parameter is the fuel density-impulse, which is defined as the product of the fuel specificimpulse and its density. It determines the necessary fuel tank volume, and therefore relates to the overall vehicle dry mass (which among other things relates to vehicle cost).
As explained in the American Institute of Aeronautics and Astronautics (AIAA) Paper No. 79-0878 (May 1979) entitled "Dual-Fuel Propulsion: Why it Works, Possible Engines, and Results of Vehicle Studies", by James A. Martin and Alan W. Wilhite, it is advantageous to use two fuels in any vehicle stage that operates from the earth surface to earth orbit. A high density-impulse fuel, such as a hydrocarbon, should be used in the first part of the vehicle trajectory. This is because a large fraction of the vehicle propellant mass is required to gain a small fraction of the vehicle final velocity. Therefore minimizing tank volume is important in this stage of the trajectory.
A high specific-impulse fuel, such as hydrogen, should be used in the second half of the vehicle trajectory. This is because the specific-impulse of the fuel has an increased effect on the vehicle performance at high velocities. Therefore a high specific-impulse fuel is advantageous later in the trajectory when vehicle velocities are high.
One way of using two fuels in a single stage is to equip the stage with a dual-fuel rocket engine. This type of engine uses two different types of fuel, such as hydrogen and a hydrocarbon, at different points in a rocket flight. Many variations of such an engine have been studied, many of which are described in the American Institute of Aeronautics and Astronautics (AIAA) Paper No. 87-1941 (June 1987) entitled "Space Transportation Main Engines for Single-Stage Vehicles", by James A. Martin. These include Dual-Expander engines, that have an inner thrust chamber and coannular outer thrust chamber that share a common exhaust nozzle, and Dual-Bell engines, that have two thrust chambers and two exhaust nozzles within a single engine. These tend to be very complex.
An engine concept called the "Dual Fuel/Single Bell", studied by Pratt & Whitney Aircraft of United Technologies Corporation, is particularly pertinent to the present invention. A combination of oxygen, hydrogen, and a hydrocarbon are all used to operate the engine. Hydrogen is used to cool the thrust chamber and drive the turbines, the hydrocarbon is used to cool the exhaust nozzle, and both are used as fuel. In the engine first mode of operation, the hydrocarbon is the primary fuel, with just enough hydrogen used to cool the thrust chamber and drive the turbines. In the engine second mode of operation, the hydrogen is the primary fuel, with just enough hydrocarbon used to cool the exhaust nozzle. The hydrocarbon and hydrogen are mixed before entering the combustion chamber.
The present invention provides an improved method of combining the two fuels in a dual-fuel engine, as compared with the prior art. Greater exhaust nozzle cooling capacity, lower fuel temperatures, and maximum efficient use of each fuel are advantages the present invention provides over prior art.